Semiconductor chips include bond pads that are electrically connected to external circuitry in order to function as part of an electronic system. The external circuitry is typically a lead array such as lead frame or a support substrate such as a printed circuit board. Electrical connection between the chip and the external circuitry is often achieved by wire bonding, tape automated bonding (TAB) or flip-chip bonding. For instance, with flip-chip bonding, ball grid array (BGA) packages contain an array of solder balls to mount on corresponding terminals on a printed circuit board, and land grid array (LGA) packages contain an array of metal pads that receive corresponding solder traces mounted on corresponding terminals on a printed circuit board.
Semiconductor packages may include multiple chips that are stacked to provide a compact high-density structure. The upper chip is often flip-chip bonded to the lower chip or interconnect metallization using solder balls. Semiconductor packages may also include a flexible conductive ribbon with an aluminum layer that is welded to the chip and the lead using wedge bonding. The aluminum layer provides high current capability, low spreading resistance and reliable bonds. However, the aluminum layer does not provide a suitable interconnection for a stacked upper chip since the wedge bond has rough, uneven top surface with sharp peaks and valleys that does not accommodate solder reflow. As a result, the upper chip would be attached to the ribbon using a solder ball that makes a poor reliability bond with the aluminum layer.
Therefore, there is a need for a semiconductor package that includes multiple chips attached to a ribbon in a reliable and cost-effective manner.